In known illumination devices, by way of example, a transflexive or transmissive liquid crystal display is surface-illuminated via a light guide to provide the illumination. In this case, a phototransistor is used to control the brightness of the display disposed in the light flux and regulates the illumination device as a function of the light flux. In European Published Patent Application No. 0 115 575 A3 a light guide is proposed which is constructed with two mutually opposite end faces and detects both the light incident through the windscreen of a motor vehicle and the light from the observation space in front of the dashboard. The total light which falls onto a milky white plastic body of the light guide is detected on laterally arranged sensors. With its relatively large dimensions, the light guide is arranged behind the windscreen in the region of the dashboard cover of a motor vehicle. Said light guide requires a relatively large amount of space, which is needed in this region for other electrical devices. Furthermore, the arrangement of the light guide behind the windscreen must be taken into consideration even when designing the passenger compartment, so that certain impediments to design freedom are imposed on the designer.